1) Field of the Invention
The present invention relates to an endoscope system that is provided with a photographing optical system using a solid-state image sensor.
2) Description of Related Art
In recent years, electronic endoscopes using solid-state image sensors such as compact CCDs have become popular. Since such an endoscope allows a plurality of persons to observe lesion in a body cavity via a TV monitor, examination and diagnosis can be made by a plurality of medical doctors. Also, it is greatly advantageous to patients also in that they can view their own lesion while being informed of doctor""s diagnosis. Of such endoscopes, so-called xe2x80x9cmagnifying endoscopexe2x80x9d, which is used for near observation of lesion to facilitate examination of its minute structure for the purpose of determining the degree of infiltration of the minute lesion or the regional extent to be subjected to incision, has drawn particular attention most recently.
Some of optical systems for the magnifying endoscopes are disclosed, for example, in Japanese Patent Publication (KOKOKU) No. Sho 61-44283 and Japanese Patent Application Preliminary Publication (KOKAI) No. 4-218012. Each of these optical systems are three- or four-unit-type optical systems and are able to provide high magnification, while involving a problem in that the entire length of the optical system is considerably long, to cause bulkiness of the insertion section of the endoscope and thus to impose a great burden on a patient.
Also, such a magnifying endoscope of prior example has a drawback in that it has an extremely narrow depth of field and thus has poor operability.
Here, brief explanation is made of the definition of depth of field. FIG. 16 is a sectional view of an endoscope optical system taken along the optical axis. As shown in the drawing, placing a CCD at a position where an image I of an object O is formed makes it possible to obtain an in-focus image. If the object O is moved toward the endoscope to the position Oxe2x80x2, the image I is shifted to be formed at a position Ixe2x80x2. On the other hand, if the object O is moved away from the endoscope to a position Oxe2x80x3, the image I is shifted to be formed at a position Ixe2x80x3. Where the position of the CCD is fixed, the image Ixe2x80x2 or the image Ixe2x80x3 at the position of the CCD becomes a circle of confusion with a diameter xcex4, to cause an out-of-focus image. However, if the resolution of the CCD is greater than the diameter xcex4 of the circle of confusion, image quality is determined by the resolution of the CCD and, accordingly, the object-position range from Oxe2x80x2 to Oxe2x80x3 seems to be in focus. This range is called xe2x80x9cdepth of fieldxe2x80x9d, where a distance Xn from the optical system to the point Oxe2x80x2 is defined as the nearest distance in the range of the depth of field, while a distance Xf from the optical system to the point Oxe2x80x3 is defined as the farthest distance in the range of the depth of field.
In this case, the following equation is true:
|1/Xnxe2x88x921/Xf|=2xcex4Fno/fL2xe2x80x83xe2x80x83(1)
where the effective aperture ratio (F number) is represented by Fno and the focal length of the optical system is represented by fL2.
Here, the depth of field D is given by:
D=Xfxe2x88x92Xnxe2x80x83xe2x80x83(2)
If we regard Xn, Xf as two values of the variable X of the function 1/X=Y, we can view a value of the operation on the left of Equation (1) in the form of a difference in Y direction between two points that lie on the curve of 1/X=Y shown in FIG. 17. If the value of 2xcex4Fno/fL2 is constant, a smaller value of Xn, or the near observation condition yields a smaller depth of field D1.
To be specific, the depth of field of a magnifying endoscope in the magnifying mode is as narrow as 2 mm or 3 mm. Such a specification would require, for example, an operator to perform subtle manipulation on the order of 1 mm while the endoscope being inserted into the large intestine over lm deep. In short, it requires a lot of skill to manipulate the endoscope (Problem 1).
Also, in the conventional magnifying endoscope system, the same image processing is performed in the normal observation mode and in the magnifying observation mode. In general, when a tissue is magnified, minute lesion in it also is observed as magnified and thus the image provided for observation would contain a large amount of low-frequency components. In contrast, since an endoscopic image in the normal observation mode shows a web pattern of fine blood vessels at a low magnification, the image provided for observation would contain a large amount of high-frequency components. Therefore, if the image processing that is optimized for the magnifying observation mode is employed in the normal observation mode, the image of the blood vessels collapses to be hardly observable (Problem 2).
Furthermore, according to the conventional magnifying endoscope, a part of the optical system located 1 m or more away is driven via a wire for switching between the normal observation mode and the magnifying observation mode. No medical doctor but the operator can know the magnification currently selected and thus there may be difference in recognition regarding the size of the lesion among them. Resultantly, it is difficult for them to make a discussion (Problem 3).
The present invention has been made in consideration of the above-mentioned problems involved in the conventional art. An object of the present invention is to provide an endoscope system which is made compact so as to impose less burden on a patient, is provided with a wide depth of field so as to allow even a person without considerable skills to easily manipulate the system in the magnifying observation mode, is able to improve accuracy of diagnosis information by enhancing seeming resolution regarding the magnified information, and is able to improve accuracy of diagnosis by facilitating discussion among a plurality of medical doctors.
In order to attain the above-mentioned object, the endoscope system according to the present invention is configured to photograph an image obtained in the normal observation mode and an image obtained in the near observation mode on a solid-state image sensor by controlling a focus adjustment means which changes the focal length and the working distance and to display the image on a display unit, wherein the following conditions are satisfied:
WDwide greater than WDtelexe2x80x83xe2x80x83(3)
fwidexe2x89xa7ftelexe2x80x83xe2x80x83(4)
where fwide is a focal length in the normal observation mode, WDwide is a working distance in the normal observation mode, ftele is a focal length in the near observation mode, and WDtele is a working distance in the near observation mode.
In general, where fL is a focal length of an optical system, fF is a front-side focal length of the optical system and z is a distance from the surface of the optical system to an object, magnification xcex2 of the optical system is given by:
xcex2=xe2x88x92fL/(fF+z)xe2x80x83xe2x80x83(5)
Accordingly, for the purpose of securing as large a magnification xcex2 as possible, it is necessary to set the distance z as short as possible or to set the focal length fL as long as possible.
On the other hand, as explained above regarding Problem 1, if the distance z from the surface of the optical system to the object is set short or the focal length fL is set long, the depth of field becomes narrow, to make the system less operable. Condition (3) is specified, according to the present invention, so that the requirement for the magnification and the requirement for the depth of field are compatibly satisfied. This is a minimum necessary condition for securing a sufficiently high magnification. In performing magnifying observation, a medical doctor would spread coloring over lesion so that the lesion is viewed in high contrast. However, this treatment would attenuate light reflected back from the object, sometimes to make the visual field dim. If only Condition (3) is satisfied, this problem of dimness can be dealt with, because an illumination lens also can be located near the object, to improve brightness.
Condition (4) is specified, according to the present invention, for the purpose of securing as wide a depth of field as possible. As is known from Equation (1), depth of field increases with decreasing focal length fL in inverse proportion to the square of fL. Therefore, the focal length in the magnifying observation mode is set to be small to satisfy the condition fwidexe2x89xa7ftele, for the purpose of widen the depth of field and, simultaneously, the endoscope is positioned near the object to satisfy Condition (3) for the purpose of increasing magnification. In this way, simultaneous satisfaction of Condition (3) and Condition (4) can provide a magnifying endoscope that achieves high magnification and wide depth of field for convenience in use.
Also, an endoscope system according to the present invention is provided with a control device that supplies a control signal based on a signal from a solid-state image sensor to a focus adjustment means, wherein, on the basis of the control signals, control of switching between image processing modes is made in accordance with the situation of the optical system.
In general, in the near observation mode, the image contains low-frequency components because a pattern peculiar to the lesion is observed with a high magnification. Therefore, enhancing the low-frequency components facilitates determination of the pattern of the lesion and thus is preferable.
On the other hand, the endoscopic image in the normal observation mode contains high-frequency components because a web pattern of minute blood vessels is photographed with a low magnification. Therefore, in the normal observation mode, enhancing the high-frequency components facilitates determination of the web pattern of the blood vessels and thus is preferable.
Conventionally, only an operator who manipulates the endoscope is able to know which of the near observation mode and the normal observation mode is currently selected. Therefore, in order to optimize the above-mentioned two image processing modes in compliance with the selected observation mode, the operator has to judge the status himself and make switching manually, which is bothersome. In contrast, according to the present invention, since switching between the near observation mode and the normal observation mode is electrically controlled by the control device, determination of the selected mode is made on the basis of the control signal and switching is performed automatically. Whereby, two kinds of image processing modes for near observation and normal observation can be properly selected without confusion so as to improve accuracy of diagnosis information by enhancing seeming resolution regarding the magnified information.
Furthermore, an endoscope system according to the present invention is provided with a control device that supplies control signals based on signals from a solid-state image sensor to a focus adjustment means, a calculating device which calculates, on the basis of the control signals, optical amounts corresponding to the status of the optical system, and a display unit which displays a result of the calculation generated by the calculating device.
As described above, the optical system of the endoscope is designed to have a relatively wide depth of field. To be specific, it has an observation range varying from 5 mm wide to 100 mm wide. On the other hand, as expressed by Equation (5), the magnification of the optical system is the function of the distance z to the object and the focal length fL, and thus the optical magnification associated with the depth of field 5 mm and the optical magnification associated with the depth of field 100 mm greatly differ.
Ideally, it is desirable that the dimensions of lesion are definitely determined. However, unless the distance to the lesion is known, the optical magnification cannot be determined. In this case, measurement methods typically including triangulation are not preferred because application of such a method would result in bulkiness of the endoscope distal end section.
Detailed analysis of requirements of medical doctors"" has revealed that, besides accurate dimensions of lesion, common recognition among the medical doctors regarding magnification range of the object image currently in focus would remove misunderstandings in discussion among them and thus can improve accuracy of diagnosis.
According to the present invention, switching between the near observation mode and the normal observation mode is electrically controlled by the control device, where optical information peculiar to each of the near observation mode and the normal observation mode is calculated on the basis of the control signals so as to be displayed on the display unit as useful information. Whereby, since which of the near mode and the normal mode is the current observation mode and optical amounts, which typically include the magnification, in the current mode are available on the display, it is possible to provide an endoscope system which would preclude possible misunderstandings among the medical doctors and thus would improve accuracy of diagnosis.
This and other objects as well as features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings.